Update libpng to 1.6.43.

This fixes a handful of bugs in parsing corrupt PNGs.

Change-Id: If53c23b64680e4231bfe47b79535daa36f4f290d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/841476
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
2 files changed